Source : Webster's Revised Unabridged Dictionary (1913)

Understandingly \Un`der*stand"ing*ly\, adv.
   In an understanding manner; intelligibly; with full knowledge
   or comprehension; intelligently; as, to vote upon a question
   understandingly; to act or judge understandingly.

         The gospel may be neglected, but in can not be
         understandingly disbelieved.             --J. Hawes.

Source : WordNet®

understandingly
     adv : with understanding; "she nodded understandingly"